In recent years video image acquisition techniques, both in the visible and in the infrared, have found application in a number of areas including nondestructive evaluation, remote sensing, energy control, visual inspection, robotics, medical imaging, machine vision, etc. Efforts to improve the quality of the images produced are usually focussed on post-processing, i.e. techniques for processing the images after they are acquired and stored in a computer. While these techniques are highly developed, and are capable of improving the image quality to a considerable extent, they suffer from the drawback that they cannot add information to the image. That is, if information about some feature of the object field was not originally present in the image acquired by the camera, no amount of post-processing can put it there afterward.
When images are acquired by slower point-by-point scanning instead of using a video camera, it is often possible to improve the quality of images by using conventional signal processing techniques, as opposed to image processing techniques, on the incoming data. For instance, one common approach to making so-called thermal-wave images is to heat the object to be observed in a periodic fashion, and to use a single infrared detector focussed on one point of the object to detect the periodic heating and cooling of that point. The resulting periodic signal from the detector is then processed by a conventional lock-in amplifier, which is referenced to the periodic heat source, and which rejects that portion of the detector's signal which is not synchronous with the heat source. The result is an improved signal-to-noise ratio, and the rejection of any non-synchronous background signal. The information in this improved signal is then stored as one pixel of what will eventually be an image, and the system then moves on to acquire the next pixel. The essential feature here is the use of synchronous detection to enhance the image before it is stored in the computer, thus negating the need for post-processing. The major drawback of such a system is that it is quite slow because the data for each pixel are processed sequentially by a single lock-in amplifier, its being impractical to have a separate lock-in amplifier for each of the pixels in a 512.times.512 (262, 144) pixel image.
An example of conventional lock-in detection with a single lock-in amplifier as described above is given by U.S. Pat. No. 4,652,757, issued Mar. 24, 1987 in the name of Carver, which discloses a technique for heating the object to be observed in a periodic fashion with a pump laser, and using a single infrared detector and lock-in to detect the periodic heating and cooling of the object
U.S. Pat. No. 4,878,116, filed Jun. 2, 1988 and issued Oct. 31, 1989, invented by the same inventors of the subject application and assigned to the same assignee, discloses a vector lock-in imaging system which uses an infrared or visible video camera coupled to a real-time image processor and a computer workstation to perform phase-sensitive lock-in detection on all of the pixels of an image in parallel, thus achieving the effect of having 512.times.512 lock-in amplifiers. In this invention the processor multiplies the incoming video signal by the sine and the cosine of the reference signal in real time, and averages the two products in separate frame buffers to produce in-phase and quadrature images The result is the enhancement of the synchronous components of the image and the suppression of incoherent noise. Since the processing is performed in real time, this system is much faster than those which employ either a point-by-point scan in conjunction with a single lock-in, or those which rely solely on post-processing of images.